1. Field of the Invention
This invention relates to an electromagnetic clutch for use in controlling the operation of a compressor and the like for use in cooling and heating unit in a vehicle, and more particularly to improvements in the exciting device thereof.
2. Description of the Prior Art
For better understanding of the features of the exciting device according to the present invention, description will first be given of a prior art device of this type in conjunction with FIGS. 1 and 2, hereunder.
Referring to FIG. 1, shown at 1 for instance is a shaft of a compressor for use in a cooling and heating unit in a vehicle, and at 1a, for instance, is a fixing member such as a housing of a compressor. Shown at 2 is a hub coupled to the end portion of the shaft 1, and the hub 2 supports a disk-like movable piece 4 of a magnetic material thereon by the medium of a plate spring 3. Shown at 5 is a rotor adapted to rotate in association with the rotation of an engine, and the rotor 5 consists of a disk-like end surface 5a facing the disk-like movable piece 4, a pair of cylindrical portions 5b, 5c having large and small diameters, which portions extent in the axial direction and are integral with the end surface 5a, and a V- shaped groove B defined integrally on the outer peripheral surface of the cylinder portion having a large diameter. In addition, the rotor 5 is rotatably supported by means of a bearing 1b mounted between the fixing member la and the small diameter cylindrical portion 5b. Shown at 6 is a stator constituting an exciting device, and the stator 6 consists of a disk-like fixing portion 6a for use in securing same to part of the fixing member 4, large and small diameter cylindrical portions 6b, 6c which are integral with the fixing portion 6a and inserted into an annular space or a hollow defined by the large and small diameter cylindrical portions 5b, 5c, with a small clearance being provided therebetween, and an exciting coil 8 molded into the annular space defined by the cylindrical portions 6b, 6c. Shown at 9 is a terminal of the exciting coil 8, and the feed of an electric current to the terminal 9 provides a magnetic circuit shown by a broken line A, with the aid of the stator 6, rotor 5 and movable piece 4. In this respect, the movable piece 4 is attracted to the end surface 5a of the rotor 5 against a force of a spring 3 and transmits the rotation to a rotary shaft 1 of a compressor and the like through the medium of the hub 2, on which the movable piece 4 is secured.
A known arrangement and manufacturing method of the exciting portion are as follows: (i) a casting is machined so as to provide a stator 6 which has a F-shaped cross-section; a conductor is wound around a coil bobbin 7 to give a coil thereon; the coil bobbin 7 is inserted into an annular space defined by the large and small diameter cylindrical portions 6b, 6c; and finally a resin is impregnated into the aforesaid space according to a molding technique; or (ii) a casting is machined so as to give a member having a small diameter portion 6b which has a L-shaped cross section, as shown in FIG. 3; an insulating portion 7 is provided on the outer peripheral surface of the cylindrical portion 6b; a conductor is directly wound around the cylindrical portion 6b to give an exciting coil 8; a large diameter portion 6c is secured to the end portion of a flange portion integral with the small diameter cylindrical portion 6b; and finally a resin is impregnated around the coil 8 to thereby provide a stator 6.
As shown in FIG. 4, there is known another attempt for providing the exciting portion, in which a steel sheet is formed to give a fixing portion 6 which has a cylindrical portion adapted to be fitted on a fixing body (not shown) and a cylindrical portion 6b supporting a coil thereon; the annular yoke E is fitted on the outer periphery of the cylindrical portion 6b and then welded to part of the fixing portion 6; then the an annular coil 8 wrapped with the insulating material 7 is fitted on the outer periphery of the cylindrical portion 6b; a jig D as shown by the two-point chain line is placed on the annular coil 8; and a resin is poured around the coil 8 to mold the coil 8 around the cylindrical portion 6b.
However, the exciting devices as shown in FIGS. 2(a) and (b) suffer from disadvantages, in that they dictate the use of a great amount of man power for machining the stator 6 and the weight of the device is increased, in addition to inefficient manufacturing operations, since the coil 8 around the bobbin 7 is built in the stator 6.
On the other hand, the exciting device as shown in FIG. 3 permits efficient manufacturing operations, since the coil is directly mounted around the stator member 6 and presents a strong magnetic force due to the increase in turns of a coil to be provided within a given diameter, while the manufacture of the stator member 6 requires complicated machining or the use of a large-size press, and in addition a cylindrical portion encompassing the outer periphery of a coil should be provided, so that there result inefficient manufacturing operations, high cost and increase in the weight of the device. In addition, when a conductor is directly wound around the stator member 6, there is a danger of the coil of coming off the member 6 in the axial direction, because the portion supporting the coil thereon is freed in the axial direction as well.
Still furthermore, with the prior art device shown in FIG. 4, since a relatively thinner steel sheet is used for providing a supporting portion for a coil, so that this provides saving in weight and low manufacturing cost. However, since the coil supporting portion is freed in the axial direction as well, as in the case shown in FIG. 3, direct winding of a conductor on a coil supporting member is confronted by difficulties, and in addition, after being wound around a bobbin, the bobbin carrying the coil has to be fitted in the coil supporting portion. Furthermore, since the device is devoid of a cylindrical portion encompassing the outer periphery of the coil as shown in FIG. 3, the outer surface of the coil is supported merely with a resin, so that the outer periphery of the coil in the radial and axial directions, when molded with resin, should be supported by means of a jig in manufacture. As a result, this increases an additional manufacturing cost of the jig.
Meanwhile, with the conventional exciting devices of this type, it has been a common idea that a yoke portion supporting a coil (large and small diameter portions 6b, 6c in FIGS. 2(a), (b), FIG. 3, end FIG. 4) is an essential factor to provide a magnetic circuit A. However, according to the experiments by the inventors, a majority of magnetic flux in the stator 6 is concentrated on the large wall- thickness portion of the disk-like stator 6, so that large and small diameter cylindrical portions 6b, 6c (or the equivalent thereto) rather cause the leakage of magnetic flux, thus impairing the function of a normal magnetic circuit.
In other words, it is preferable that the stator 6 be composed of a portion providing the magnetic circuit A and a portion supporting the coil, and that the aforesaid two portions be of a shape that a accommodates the respective functions thereof.
In this sense, the device as shown in FIG. 4 provides a yoke portion which provides a magnetic circuit and which is of a thick disk-like shape, while the coil supporting portion has a relatively small thickness, so that this substantially meets the aforesaid requirements. However, as has been described earlier, there are shortcomings in the facts that the conductor is hard to be directly wound around the stator, and the jig for use at the time of molding is costly.